Metabolic energy and cytoskeletal requirements for synthesis and secretion by acini from rat mammary gland-I. Ultrastructural and biochemical aspects of synthesis and release of milk proteins

The effect of lactogenic hormones on protein synthesis and amino acid uptake in rat mammary acinar cell culture at various physiological stages

1. The activities of protein synthesis and amino acid uptake at various physiological stages were determined by the incorporation of radioactive materials ([3H]-lysine, [14C]-cycloleucine) in rat mammary epithelial cell cultures. The activity of protein synthesis and amino acid uptake was higher in early lactation than in virgin, pregnant and late lactation stages. 2. Lactogenic hormones (prolactin, hydrocortisone and insulin) treatment related with mammary growth and differentiation increased the activities of protein synthesis and amino acid uptake. But increase of these activities was different at each physiological stage. 3. The effect of prolactin and hydrocortisone on the activities were greater in virgin, pregnant and late lactation than in early lactation. And effect of insulin was greater in pregnant and early lactation than in virgin and weanling.

Mammary gland function during involution

The process of mammary gland involution occurs during the transition from a lactating to a nonlactating state. This transition phase begins after cessation of milk removal and results in changes in mammary secretion composition. Secretion volume declines during involution, as does the concentration of most milk-specific components. Lactoferrin, hydrolytic enzymes, immunoglobulins, and serum-derived components increase in concentration in the secretions during involution. Changes in mammary secretion composition may reflect changes in function of alveolar epithelial cells and have implications for the disease resistance of the gland. Histological and ultrastructural changes occurring in the gland are consistent with a decline in secretion of milk components from epithelial cells. Autophagocytic mechanisms may be involved in this decline in the lactation function. Ultrastructurally, there is little evidence for an extensive loss of epithelia in the bovine mammary gland during involution. Completion of the functional changes occurring in the gland during the process of involution may be required for the gland to redevelop fully for maximal milk yield in the subsequent lactation. Cellular mechanisms involved in mammary involution and relationships between the processes of involution and redevelopment should be areas of particular interest in the mammary function of dairy cattle.

Secretion-coupled protein degradation: studies on mammary casein

Mammary explants from midpregnant rabbits were cultured for 18 h at 37 degrees C with insulin, prolactin and cortisol. Subsequently, explants were labelled for 2 h with inorganic [32P]phosphate, L-[5-3H]proline or L-[4,5-3H]leucine, washed and chased for up to 3 h. The radiolabelling profile of [32P]casein or [3H]casein during the chase period, obtained by isoelectric focussing or immunoprecipitation indicates extensive destruction of neosynthesized casein. The extent of casein destruction in mammary explants in culture (measured after radiolabelling with L-[5-3H]proline), is inversely related to casein secretion. Least casein degradation is observed in explants after 48 h in culture when casein secretion is maximal (observed histochemically). Subsequently, when the extracellular alveolar lumen is filled with secretion products (72 h), rapid intracellular casein destruction is again observed. When the chase was carried out in the presence of drugs which inhibit degradation and/or secretion, the results indicate that secretion-coupled casein degradation is dependent on an intact functional microfilamentous-microtubular network, casein is not degraded by an autophagosome requiring process, degradation is inhibited by leupeptin, amino-acid analogue containing casein does not undergo secretion-coupled degradation and inhibition of N-glycosylation of intracellular vesicular membrane proteins prevents secretion-coupled degradation. Secretion-coupled protein destruction is discussed in relation to the post-translational regulation of the net production of secretory proteins in eukaryotic cells.

Effects of colchicine on milk yield, composition, and cellular differentiation during caprine lactogenesis

Intramammary colchicine infusion into goats at parturition reduced milk yield by 20% during the 30 day experimental period. During the first week of lactation, milk composition from colchicine-treated udder halves had elevated somatic cell numbers, serum albumin concentration and pH, while citrate concentration was lower in comparison to uninfused glands. Levels of lactose from both infused and uninfused udder halves were normal during the first week of lactation. No differences were observed in degree of alveolar development in tissue samples collected prior to treatment. Light and electron microscopy suggested that colchicine-treated udder halves consisted predominantly of undifferentiated mammary secretory cells, while uninfused udder halves appeared more cytologically differentiated. Results demonstrated that intramammary colchicine infusion at parturition temporarily altered milk composition and inhibited mammary cellular differentiation.

Ultrastructural changes in lactating tissue related to the suppression of milk secretion by concanavalin A

The plant lectin, concanavalin A (Con A) suppresses milk secretion when infused into the mammary gland or when incubated with lactating tissue in vitro. Toward defining its mode of action, we infused Con A into rat and goat mammary glands via the teats and observed effects on lactating cells. Lectin dosages were 2 and 25 mg per gland for rats and goat, respectively. Tissue samples were taken 1 and 3 h post infusion for rats and at 24 h for the goat. Control and Con A-treated tissues were observed by light microscopy and by both thin section and freeze fracture electron microscopy. In comparison to controls, Con A-treated tissues of both species exhibited alveoli with enlarged cells and relatively empty lumina; cells were distended with secretory vesicles and fat droplets. Apical plasma membranes of lectin-affected cells of the rat displayed a marked reduction in the number of microvilli, and exhibited an atypical branching and folded structure. Morphometry was employed to quantitate changes in cell and secretory product parameters in both rat and goat tissue. Microtubule numbers and distribution did not appear to be altered by Con A but considerable changes were noted in the arrangement of microfilaments associated with the secretory surface of lectin-treated epithelial cells. Various related ultrastructural changes and the role of Con A in perturbing the microfilament system are discussed.

Effect of prepartum blockade of microtubule formation on milk production and biochemical differentiation of the mammary epithelium in holstein heifers

1. Prepartum intramammary infusions of colchicine markedly reduced subsequent milk production compared with untreated mammary glands in the same animal. 2. After the first week postpartum, milk composition was similar in control and treated mammary glands. 3. Rates of fatty acid synthesis, CO2 production and protein synthesis at either 5 or 21 days postpartum were lower in mammary tissue slices taken from mammary glands treated prepartum with colchicine. 4. We conclude that an intact microtubule system is necessary for initiation of milk synthesis and secretion at parturition.

Effect of prepartum blockade of microtubule formation on ultrastructural differentiation on the mammary epithelium in holstein heifers

1. Prepartum infusion of colchicine into mammary quarters of heifers inhibited normal development of bovine mammary secretory tissue. 2. In comparison with control, tissue from colchicine-infused quarters exhibited 22.6% less alveolar luminal area and 17.1% more interalveolar connective tissue area indicating a reduction in secretory activity. 3. Cytological analysis demonstrated that undifferentiated cells predominated in treated quarters whereas the majority of cells in control quarters were fully differentiated. 4. Although control quarters reached normal histological development by the third week postpartum, tissue from colchicine-infused quarters remained relatively immature. 5. Results suggest that (1) an intact microtubule system is necessary during the prepartum period if the mammary gland is to respond normally to lactogenic stimuli and (2) transient disruption of microtubule integrity during the prepartum period irreversibly suppresses differentiation of mammary epithelia during the subsequent lactation.

Calcium and calcium ionophore A23187 alter protein synthesis and secretion by acini from rat mammary gland

1. Acini (alveoli) from lactating rat mammary gland responded to increasing levels of calcium or calcium ionophore A23187 with increased protein synthesis and secretion. 2. Inclusion of dibutyryl cyclic AMP in incubation medium with calcium or calcium ionophore decreased protein synthesis but increased rate of protein secretion . 3. Calcium ionophore increased both rates of calcium uptake by acini and levels of intracellular calcium. 4. These results suggest a relationship between calcium and cyclic AMP in milk protein synthesis and secretion.

Metabolic energy and cytoskeletal requirements for synthesis and secretion by acini from rat mammary gland-II. Intracellular transport and secretion of protein and lactose

1. Iodoacetate, 2,4-dinitrophenol, cyanide and cycloheximide inhibited protein secretion as well as synthesis by acini (alveoli) from rat mammary gland. Cytochalasin B and vinblastine inhibited protein secretion and marginally reduced protein synthesis. Colchicine was without effect on protein synthesis but inhibited secretion. 2. Intracellular protein transport was altered during incubation with metabolic and cytoskeletal inhibitors. Cycloheximide, iodoacetate. 2,4-dinitrophenol and cytochalasin B appeared to block protein synthesis on polysomes of rough endoplasmic reticulum. Vinblastine inhibited protein transport from rough endoplasmic reticulum to Golgi apparatus and colchicine appeared to cause accumulation of protein in several endomembrane fractions. 3. Iodoacetate reduced acinar lactose content but was without effect on lactose synthetase activity. Cyanide, cycloheximide and vinblastine reduced reduced lactose synthetase activity but not tissue lactose concentration. Cytochalasin B reduced glucose incorporation but was without effect on lactose content and lactose synthetase activity. Colchicine and 2,4-dinitrophenol did not alter glucose incorporation, lactose content or lactose synthetase activity. Lactose secretion was inhibited by all metabolic and cytoskeletal inhibitors examined. 4. Results indicated that sustained protein secretion depended on continued protein synthesis and that lactose secretion was coupled to protein secretion.